(a) Field of the Invention
The present invention relates to an interference canceling device and method in a mobile communication system. More specifically, the present invention relates to a group-wise interference canceling method in a CDMA (code division multiple access) system for reducing multiple access interference.
(b) Description of the Related Art
In the CDMA system, since each mobile station uses a particular spreading code to receive mixed signals transmitted through an identical frequency band, the performance of the system is lowered because of multiple access interference generated when the received signals are despread. Hence, a subtractive interference cancellation method for removing other users' signals, that is, interference signals, from the received mixed signals of a plurality of users has been proposed so as to increase performance and capacity of the system. The subtractive interference cancellation method includes the SIC (successive interference cancellation) method for canceling the interference signals one by one, and the PIC (parallel interference cancellation) method for concurrently canceling them. The SIC method improves the system performance but causes time delays, and hence, it is difficult to realize the SIC method in an actual system. The PIC method deteriorates the performance in a multi-path fading environment having substantial channel variations. To compensate for the demerits of the subtractive interference cancellation method, methods for dividing the interference into groups and canceling the interference for each group have been proposed.
The methods for canceling the interference of the respective groups include a method for grouping signals according to signal power and sequentially selecting the signals in order of start from a group of high signal power to cancel the interference; a method for providing weights to the respective groups and concurrently canceling the interference; and a method for grouping the signals for each finger according to the power of the received signals, selecting groups in order of from the group of high signal power, subtracting a corresponding regeneration signal to cancel the interference, adding the regeneration signal again in a next interference canceling stage, and repeating these stages for each interference stage.
The above-described conventional interference canceling methods divide the signals into groups in consideration of the power of the transmission signals, data speeds in multiple transmissions, intensities of received signals, or output values of a maximal ratio combiner (MRC). However, in the multiple path fading channel environment, the signal power at the time of receiving the signals may be very weak even when strong signals have been transmitted, and so if these signals are selected for interference cancellation, the performance is deteriorated. Also, respective signals have different bit lengths in the case of multiple data speed transmissions, and signals of high instant bit energy have higher reliabilities than those of high instant power at the time of receipt, and accordingly, consideration of received signal powers problematically generate incorrect regeneration signals for subtraction.
Further, the conventional grouping methods require predefinition of a number of signals for each group for canceling the interference, or the number of signals for each group is predefined according to transmission speeds. However, an optimal value of the number of signals for each group is not previously known, and when the number of signals for each group is predefined according to the transmission speed, accurate grouping may not be executed in the multi-path fading channel environment.